Jeff Wisnia writes:

Though, if push came to shove I'm sure one could use a run capacitor of
the correct capacitance rating to do a start capacitor's job, but not
the other way around, huh?

Probably. The run capacitor would be too large to fit the space
provided for a start capacitor of the same voltage and capacitance
rating, but it ought to work fine electrically. The start capacitor
used as a run capacitor would soon overheat.

Dave